1911 Encyclopædia Britannica/Crassulaceae

CRASSULACEAE, in botany, a natural order of dicotyledons, containing 13 genera and nearly 500 species; of cosmopolitan distribution, but most strongly developed in South Africa. The plants are herbs or small shrubs, generally with thick fleshy stems and leaves, adapted for life in dry, especially rocky places. The fleshy leaves are often reduced to a more or less cylindrical structure, as in the stonecrops (Sedum), or form closely crowded rosettes as in the house-leek (Sempervivum). Correlated with their life in dry situations, the bulk of the tissue is succulent, forming a water-store, which is protected from loss by evaporation by a thickly cuticularized epidermis covered with a waxy secretion which gives a glaucous appearance to the plant. The flowers are generally arranged in terminal or axillary clusters, and are markedly regular with the same number of parts in each series. This number is, however, very variable, and often not constant in one and the same species. The sepals and petals are free or more or less united, the stamens as many or twice as many as the petals; the carpels, usually free, are equal to the petals in number, and form in the fruit follicles with two or more seeds. Opposite each carpel is a small scale which functions as a nectary. Means of vegetative propagation are general. Many species spread by means of a creeping much-branched rootstock, or as in house-leek, by runners which perish after producing a terminal leaf-rosette. In other cases small portions of the stem or leaves give rise to new plants by budding, as in Bryophyllum, where buds develop at the edges of the leaf and form new plants.

Stonecrop (Sedum acre) slightly reduced. 1, Horizontal plan of
arrangement of flower of stonecrop; 2, flower of Sedum rubens.

The order is almost absent from Australia and Polynesia, and has but few representatives in South America; it is otherwise very generally distributed. The largest genus, Sedum, contains about 140 species in the temperate and colder parts of the northern hemisphere; eight occur wild in Britain, including S. Telephium (orpine) and S. acre (common stonecrop) (see fig.). The species are easily cultivated and will thrive in almost any soil. They are readily propagated by seeds, cuttings or divisions. Crassula has about 100 species, chiefly at the Cape. Cotyledon, a widely distributed genus with about 90 species, is represented in the British Isles by C. Umbilicus, pennywort, or navelwort, which takes its name from the succulent peltate leaves. It grows profusely on dry rocks and walls, especially on the western coasts, and bears a spike of drooping greenish cup-shaped flowers. The Echeveria of gardens is now included in this genus. Sempervivum has about 50 species in the mountains of central and southern Europe, in the Himalayas, Abyssinia, and the Canaries and Madeira; S. tectorum, common house-leek, is seen often growing on tops of walls and house-roofs. The hardy species will grow well in dry sandy soil, and are suitable for rockeries, old walls or edgings. They are readily propagated by offsets or by seed.

The order is closely allied to Saxifragaceae, from which it is distinguished by its fleshy habit and the larger number of carpels.